PDPs (plasma display panels) have been widely used in the field of display devices in the past, and a larger screen, higher quality, and lower cost have been required of PDPs in recent years.
A three-electrode surface discharge type, which is made up of a front panel comprising a sustaining electrode and a scanning electrode formed on a glass substrate, and a rear panel comprising an address electrode formed on a glass substrate, with these panels put together, is generally coming to the main current of PDP.
An inert gas is enclosed between the front and rear panels, and voltage is applied between the scanning electrode and the address electrode to create a discharge, whereupon the inert gas is plasmatized so that ultraviolet rays are emitted. If a fluorescent film is disposed at the location where the emitted ultraviolet rays are irradiated, the ultraviolet rays will cause the fluorescent film to luminesce and emit colored light.
A dielectric film is usually formed over the sustaining electrode and the scanning electrode; and an MgO protective film is formed over this dielectric film to protect it.
When AC voltage is applied to the scanning electrode and the sustaining electrode to sustain discharge, cations generated by plasmatizing the inert gas are incident on the scanning electrode side and the sustaining electrode side, respectively. However, the scanning electrode and the sustaining electrode, and the dielectric film over these electrodes, are protected against the cations by the protective film.
Therefore, the dielectric film is not damaged by the plasma, and the sustaining electrode and scanning electrode are maintained in a state of being insulated by the dielectric film; and furthermore, since there is no change in the electrostatic capacitance of the dielectric film, the electrical characteristics of the plasma display panel are maintained.
In order to increase the brightness of a PDP in response to the growing requirements for higher performance in recent years, a method has been proposed in which luminescent intensity is increased by raising the concentration of xenon in a mixed gas of neon and xenon, which is usually used as the inert gas, from the conventional level of about 5% to 10% or higher.
However, when the protective film is made of MgO, if the xenon concentration in the inert gas is raised, the discharge voltage goes up, anti-sputtering property of the protective film decrease, and the function as a protective film subsequently declines. Therefore, there has been a problem of shortening a panel service life. There also has been a problem of increasing in cost of manufacturing a PDP drive control system because the driver circuit for driving the PDP must be able to handle higher voltage.
The discharge voltage of a PDP depends on the secondary electron emission coefficient of the protective film. It has been proposed that the discharge voltage is possible to be reduced if an oxide of an alkaline earth metal, which has a smaller work function than MgO, is used as the protective film.
For instance, Japanese Laid-Open Patent Application 2002-231129 (patent document 1) introduces SrO, CaO, BaO, SrO+BaO, BaO+CaO, and SrO+CaO+BaO as protective films.
These protective films are less resistant against sputtering by cations during discharge than MgO is, and are disadvantageous in terms of PDP service life such that technology attempts to solve the problem of shorter service life by forming a protective film not only on the front panel of a PDP, but also on the rear panel where at least a fluorescent film is formed.
However, a problem encountered on carrying out the above prior art is that increasing the protective film formation steps drives up the cost of manufacturing a PDP. Another problem that remains to be solved is that aging treatment of the protective film, which is considered necessary at the outset of discharge, takes as long or longer than with MgO. The present invention provides a protective film that has superior anti-sputtering property and lower discharge voltage than an MgO film, and a method for forming this protective film, as well as a PDP manufacturing method for shortening the time for the initial discharge aging treatment of the PDP in which this protective film is used.